The present invention relates to communications methods and apparatus, and more particularly, methods and apparatus for processing communications signals.
FIG. 1 illustrates a conventional receiver as may be used in a code division multiple access (CDMA) communications system. The receiver includes an antenna 10, which receives signals, and a radio processor circuit 11 that filters, amplifies and converts the received signals to a suitable form for processing, such as complex numerical sample values. The conventional receiver further includes a channel estimator 12 that correlates received signal samples with known symbols 14 (stored or generated locally) to determine channel estimates that are provided to a decoder 13. The decoder 13 processes the signal samples together with the channel estimates to extract information. For example, the decoder 13 may be a RAKE receiver as described in U.S. Pat. No. 5,305,349 to Dent et al. and U.S. Pat. No. 5,572,552 to Dent et al.
In typical conventional CDMA cellular communications systems complying with the IS-95 standard, a base station transmitter transmits a stream of known symbols referred to as a pilot code on the same channel as other, information bearing symbols, with the channel being modulated according to a spreading code. The transmitted signal is received over multiple paths. The channel estimator 12 correlates the received signal, which includes the pilot code and other information in additive superposition, and low-pass filters the resultant complex correlation to obtain channel estimates, which, in principle, are then known continuously. The received signal is correlated with different delays between the pilot code and the received signal to obtain channel estimates for each of the multiple paths. The received signal is also correlated with other codes carrying information to be decoded. The results of the correlations are multiplied by the conjugate of the channel estimate for the same delay, and the results are added to coherently combine the multipath signals. This can provide an optimum combination when white noise dominates.
U.S. Pat. No. 5,572,552 to Dent et al. illustrates that the well-known RAKE receiver for receiving and decoding CDMA signals through a multipath channel may not be optimal for CDMA cellular systems in which most of the interference with a desired signal at a mobile terminal comes from signals transmitted by the same cell site that transmits the desired signal, a type of interference commonly referred to as “own cell” interference. In the presence of such own-cell interference, an optimum receiver may be an inverse channel equalizer.
The aforementioned U.S. Pat. No. 5,572,552 also describes that, in a case in which there is mixed own-cell and other-cell interference, i.e., interference from signals transmitted in other cells, optimum reception may be achieved by combining despread values using combining weights that partly resemble prior art RAKE taps and partly resemble an inverse channel equalizer. Such a receiver has been referred to as a generalized RAKE (G-RAKE) receiver and may include variations in which values are despread or despread values are combined to achieve interference cancellation. Combining coefficients in such a receiver may be computed from estimates of the multipath channel coefficients together with autocorrelation coefficients for the interference. A G-RAKE receiver may also incorporate techniques introduced in U.S. Pat. No. 6,363,104 to G. E. Bottomley entitled, “Method and apparatus for interference cancellation in a RAKE receiver,” filed Oct. 2, 1998; U.S. Pat. No. 6,714,585 to Wang et al. entitled, “RAKE combining methods and apparatus using weighting factors derived from knowledge of spread spectrum signal characteristics,” filed Jun. 25, 1999; U.S. Pat. No. 6,683,924 to Ottosson et al. entitled, “Apparatus and methods for selective correlation timing in RAKE receivers,” filed Oct. 19, 1999; and U.S. Pat. No. 6,922,434 to Wang et al. entitled, “Apparatus and methods for finger delay selection in RAKE receivers,” filed Apr. 30, 2001. The G-RAKE may use strategies for finger placement and combining weight design that suppress interference. The weights may be determined based on channel estimates and an estimate of the overall noise correlation matrix across fingers. Another example of a demodulator is the multi-stage RAKE introduced in U.S. Pat. No. 6,801,565 to Bottomley et al. entitled, “Multi-stage RAKE combining methods and apparatus,” filed Jun. 25, 1999.
In wideband CDMA (W-CDMA) systems under current development in Japan and Europe, trade-offs between the amount of transmitter power devoted to sending pilot codes or symbols to facilitate channel estimation and the amount of transmitter power devoted to transmitting user data often result in channel estimation error. A consequence of such error may be uncertainty as to which of a G-RAKE , traditional RAKE or other method of combining despread values is optimum.
U.S. Pat. Nos. 5,557,645 and 5,619,513 to Dent describe that the number of states in a maximum least square estimation (MLSE) equalizer can be expanded beyond the number needed to deal with the multipath channel memory in order to accommodate more estimates of the multipath channel coefficients. U.S. Pat. No. 6,520,910 to Bottomley et al., entitled “Baseband processor with look-ahead parameter estimation capabilities” and filed Oct. 25, 1999, describes that the number of states in a Viterbi MLSE equalizer can be further expanded to encompass extra channel estimates based on hypotheses of future symbols to be decoded.
U.S. Pat. No. 5,230,003 to Dent et al. describes decoding techniques for distinguishing between differently coded data symbols, based on a decoding quality indicator for decoding with each type of code. U.S. Pat. No. 5,841,816 to Croft et al. describes selecting demodulation techniques from a repertoire of demodulation algorithms including diversity combining algorithms using multiple antennas and receivers, non-diversity techniques using a single antenna and receiver, coherent and non-coherent techniques, techniques employing equalizers to compensate for multipath propagation and techniques not involving equalizers, and forward-time, backward-time and mixed bidirectional demodulation algorithms. According to these proposed techniques, the selection of the appropriate algorithm for demodulating is made based on some measurable characteristic of the signal, in particular, on the quality or signal-to-noise ratio of channel estimates made with the help of known signal patterns embedded in the signal. Correlations between different patterns of the measurable characteristics and the choice of algorithm that, on average, provides the best decoding for each pattern may be determined off line by simulation during design and then built into equipment. The optimum demodulation algorithm may then be selected based on the observed pattern of measurable signal characteristics for each instance of demodulation.
Unfortunately, according to such an approach, it may happen that selection of an demodulation algorithm is non-optimum for a given instant, even if the selection was optimum on average. This may occur, for example, when the measured signal characteristics are not indicative of the best instantaneous choice of algorithm.